An electronic device such as a power supply or an inverter to output high electric power typically includes a converter circuit which according to a control signal of a PWM circuit, amplifies the electric power to be delivered to a load. In general, such a converter circuit includes a switching circuit which converts a DC voltage signal into an AC voltage signal under control of the PWM signal. An example of the switching circuit which converts a DC voltage signal into an AC voltage signal may include a half-bridge switching circuit, a full-bridge switching circuit and a push-pull switching circuit. Among these, the full-bridge switching circuit which includes a monopolar power source and four switching elements is a switching circuit which is used for large output capacities and high output voltages.
A single channel full-bridge switching circuit having a single channel is sometimes called a “H-bridge” since a total of 4 switching elements are arranged in the form of an H with a primary coil of a transformer interposed therebetween. Typically, the full-bridge switching circuit controls the output voltage by changing the ON pulse width of each switching element while alternately making the switching elements arranged in the form of an H conductive.
With the increase in size of various electronic devices, two-channel full-bridge switching circuits have been increasingly used in place of single channel full-bridge switching circuits. A conventional two-channel full-bridge switching circuit is generally constructed using two single-channel full-bridge switching circuits, including a total of 8 switching elements. Since each channel in the two-channel full-bridge switching circuit needs to be separately controlled, an integrated circuit (IC) has to control operation of the respective 8 switching elements. However, since such two-channel full-bridge switching circuits have many switching elements, the configuration is complicated which increases the cost and size of the circuit.
Relating to the above problem, in an effort to reduce the number of switching elements, a two-channel full-bridge switching circuit having 6 switching elements has been developed. Such a switching circuit is configured to short-circuit one terminal of each primary coil of the transformers for two channels to form a common node, and use two switching elements connected to the common node commonly for both channels. The switching circuit having such configuration has to control the channels separately and, therefore, employs a phase-shift method to obtain a desired result by controlling the ON/OFF time of the remaining switching elements for circuit operation based on the ON/OFF time of a switching element connected to the common node and a corresponding potential of the common node.
Therefore, there is a need for a two-channel full-bridge switching circuit having 6 switching elements, which is capable of controlling channels separately without using a complicated phase-shift method for control.